Melody, hydrogen peroxide is sometimes used as a disinfectant because it is a strong oxidising agent. Some people use it to treat skin wounds because it may kill bacteria in those wounds. This is controversial - the third page in this link discusses this; http://www.medscape.com/viewarticle/456300.

I don't know whether or not it, applied to the skin, may help with psoriasis - a PubMed search does not say much either way. This articles is interesting - it suggests that manuka honey (a good NZ product!) may help with psoriasis, possibly because it can produce hydrogen peroxide; http://www.ncbi.nlm.nih.gov/pubmed/24305429.

I doubt that taking a small amount (a few mL; taking more could be toxic) of hydrogen peroxide by mouth will be of any use, for two reasons. It will rapidly get diluted into the ~30-40 litres of water in the human body to the extent that it will have no effect. Also, it will rapidly be inactivated by catalase, which is a very active enzyme; one molecule of it can break down five million molecules of hydrogen peroxide every minute